This project demonstrates a two-way cross-chain handshake between two smart contracts deployed on different networks.
A relayer service listens for events and sends transactions across chains to complete the handshake..

• contract a (sepolia): 0xddA72c449E9e8Cf5b440859c6AA78d8E1803FAc3
• contract b (arbitrum sepolia): 0x8c7A0E5A49f6f4ca7AA8EE10933Ab5E7bCE85FeF

• sepolia rpc: https://eth-sepolia.g.alchemy.com/v2/cuJGeZ26qywahhkli6epc2XqIOoZJhFF
• arbitrum sepolia rpc: https://arb-sepolia.g.alchemy.com/v2/AJAOv3C3nutHF-2duNghabWWPgzD1hxw

• initiated from contract a with session id 785
• transaction hash: 0x8c3025777753e38b5073e4e69097c32639588d4b91965e1c4a87856bba8d5cbd

1. fill the .env file presnt in ./go-relayer directory with the following variables:

SEPOLIA_RPC=https://eth-sepolia.g.alchemy.com/v2/cuJGeZ26qywahhkli6epc2XqIOoZJhFF
RPC_B=https://arb-sepolia.g.alchemy.com/v2/AJAOv3C3nutHF-2duNghabWWPgzD1hxw
RELAYER_PVT_KEY=your_private_key
CONTRACT_A_ADDR=0xddA72c449E9e8Cf5b440859c6AA78d8E1803FAc3
CONTRACT_B_ADDR=0x8c7A0E5A49f6f4ca7AA8EE10933Ab5E7bCE85FeF
CHAIN_IDA=11155111    # sepolia chain id
CHAIN_IDB=421614      # arbitrum sepolia chain id

2. build and run the relayer:

go build ./cmd/relayer
./relayer

any user with a funded sepolia wallet can initiate a handshake by calling initiateHandshake on contract a.

export CONTRACT_A_ADDR=0xddA72c449E9e8Cf5b440859c6AA78d8E1803FAc3
export SEPOLIA_RPC=https://eth-sepolia.g.alchemy.com/v2/cuJGeZ26qywahhkli6epc2XqIOoZJhFF
export YOUR_PRIVATE_KEY=your_private_key_here
export SESSION=

cast send $CONTRACT_A_ADDR "initiateHandshake(uint256)" $SESSION \
    --rpc-url $SEPOLIA_RPC \
    --private-key $YOUR_PRIVATE_KEY

this will emit a syn event, and the relayer will automatically complete the handshake across the two networks.

Currently if the relayer restarts it forgets which sessions it has already handled which could cause duplicate transactions.

Solution:
Integrate a simple LevelDB instance to store:

• Last processed block numbers
• Session IDs that have already been relayed

Right now the relayer requires manual environment setup.

Solution:

• Add a Dockerfile and docker-compose.yaml to simplify deployment.
• Add tests to validate the entire relayer flow during CI builds.

If a transaction fails due to issues like RPC errors, nonce gaps, or mempool reorgs, the relayer currently just logs the error and moves on.

Solution:
Use a retry mechanism such as go-retry or implement a simple exponential backoff system to retry failed transactions automatically.